Hydraulic fracturing is a method of using pump rate and hydraulic pressure to fracture or crack a subterranean formation. Once the crack or cracks are made, high permeability proppant, relative to the formation permeability, is pumped into the fracture to prop open the crack. When the applied pump rates and pressures are reduced or removed from the formation, the crack or fracture cannot close or heal completely because the high permeability proppant keeps the crack open. The propped crack or fracture provides a high permeability path connecting the producing wellbore to a larger formation area to enhance the production of hydrocarbons.
The development of suitable fracturing fluids is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and shear rates which can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids which have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide is used, which may or may not be crosslinked. The thickened or gelled fluid helps keep the proppants within the fluid during the fracturing operation.
While polymers have been used in the past as gelling agents in fracturing fluids to carry or suspend solid particles in the brine, such polymers require separate breaker compositions to be injected to reduce the viscosity. Further, the polymers tend to leave a coating on the proppant even after the gelled fluid is broken, which coating may interfere with the functioning of the proppant. Studies have also shown that “fish-eyes” and/or “microgels” present in some polymer gelled carrier fluids will plug pore throats, leading to impaired leakoff and causing formation damage. Conventional polymers are also either cationic or anionic which present the disadvantage of likely damage to the producing formations.
Aqueous fluids gelled with viscoelastic surfactants (VESs) are also known in the art. VES-gelled fluids have been widely used as gravel-packing, frac-packing and fracturing fluids because they exhibit excellent rheological properties and are less damaging to producing formations than crosslinked polymer fluids. VES fluids are non-cake-building fluids, and thus leave no potentially damaging polymer cake residue.
Viscoelastic surfactant (VES) gelled aqueous fluids have excellent friction loss reduction properties below a certain, but often undefined critical generalized Reynold's number. Above this critical generalized Reynold's number, as the generalized Reynold's number increases, the Fanning friction factor increases and thus, friction pressure increases rapidly to values that cause surface pumping pressure to approach preset limits.
It would thus be desirable if the VES-gelled fluid could be modified in some way that increases this critical generalized Reynold's number to slow the rate of friction pressure increase so that additional friction losses may be avoided or minimized at the same flow rate and/or pumping horsepower.
In general, friction reducers are known additives, typically in slurry or liquid form, used to reduce the friction forces experienced by tools and tubulars in the wellbore. Friction reducers are routinely used in horizontal and highly deviated wellbores where the friction forces limit the passage of tools along the wellbore.
In a different discipline and technological art, drag reducing agents are used in the pumping of hydrocarbons long distance to reduce the drag of the hydrocarbon, typically crude oil, as it travels through a pipeline, as well as to reduce the horsepower requirements necessary to pump the hydrocarbon. Such drag reducing agents or DRAs are typically hydrocarbon-soluble polymers. Conversely, water-soluble polymers are known to reduce the drag and horsepower requirements involved in pumping aqueous fluids, typically water, through pipelines.